The body's immune system, in other words, overreacts. As is typical in cases of contact dermatitis (the class of rashes you get from touching chemical irritants), it's the defense's heavy artillery -- and not the invader's puny assault -- that causes all the problems.
The process goes like this: Within as few as five minutes of contact, urushiol penetrates the skin, breaking down into chemical derivatives that are easily diffused through the dermal layers. These simpler chemicals bind themselves to skin proteins, changing the cells' appearance. But the disguise doesn't fool the body's T-cells, white blood cells that circulate through the body warding off infection.
Recognizing the invaders (without waiting around to see if they pose any real harm), the T-cells mount a vigorous attack. They migrate to the lymph nodes, where they multiply and produce urushiol-specific antibodies, including inflammatory chemicals like histamines. It's this reaction that produces the ailment's telltale itch.
In case that's not enough defense, the T-cells remember the experience, lying in wait for poison ivy's next attack, whether it's a month or a year away.
What that means is that the next time a sensitized person is exposed to urushiol, the immune system is now at alert, poised to attack. "The primed T-cells migrate back to the area of exposure and trigger the release of chemicals in the skin that cause redness and blistering," said Bruce Brod, a clinical assistant professor of dermatology at the University of Pennsylvania. "These T-cells are like good hunting dogs. Once they sniff the scent for the first time, they can easily home in on it again and again."
To the Rescue?
The study presented last fall showed that Zanfel Poison Ivy Wash, a patented mix of alcohol solubles and soap-like substances made by Zanfel Laboratories Inc. of Chicago, reduces blistering and itching from poison ivy markedly better than plain soap and water. Like Tecnu, an older rival, the product is designed to be rubbed into affected skin, where it binds to urushiol; then it's washed away.
The company-funded study, said Heller, will be submitted for publication later this year to the American Journal of Emergency Medicine.
A randomized, controlled, double-blind trial, it involved 24 participants who had both forearms rubbed with urushiol. All were then treated with Zanfel two, four and six days after exposure. Each individual acted as his own control: one arm got the Zanfel; the other, soap and water. At each interval, researchers compared skin redness, blistering, hardening and itchiness. (The first three parameters were measured using a six-point scale; itch was quantified using a visual analog scale.)
Zanfel emerged clearly "superior to plain soap and water" at every stage tested, said Heller. Zanfel's package claims to offer relief at any stage of the reaction, and often with only one wash.
Some experts are skeptical. "It's not possible for anything to remove the oil's effect a few minutes, let alone a few days, afterward," by which time allergic reactions have already taken effect, said Stephen White, a dermatologist with a private practice in Bethesda. "We know the timing in getting from point one to point two."